Combined effects of rearing temperature regime (thermocycle vs. constant temperature) during early development and thermal treatment on Nile tilapia (Oreochromis niloticus) sex differentiation

In nature, water temperature experiences daily variations known as thermocycles. Temperature is the main environmental factor that influences sex determination in most teleost fish. The purpose of this study was to examine the effects of rearing temperature (thermocycle (TC) vs. constant (CTE)) on development and a posterior thermal shock throughout the period of sex differentiation of Nile tilapia (Oreochromis niloticus). Embryos and larvae were kept under two temperature regimes: TC of 31 °C:25 °C day:night vs. CTE of 28 °C from 0 to 11 dpf. After this period, the larvae from each group were subjected to either heat treatment (HT, 36 °C for 12 days) or kept under the same rearing temperatures until 23 dpf (Control, C). Then all the groups remained at constant temperature until 270 dpf, when blood and gonads were collected. Larval samples were used to examine the expression of genes related to male (amh, ara, sox9a, dmrt1a) and female (cyp19a1a, foxl2, era) sexual differentiation. In juveniles, sex was characterized by histology, the gonadal expression of the genes involved in the sex steroid synthesis was analyzed by qPCR, and plasma testosterone (T) and estradiol (E2) levels were analyzed by ELISA. In larvae, daily TCs increased the survival rate against HT and up-regulated the expression of ovarian differentiation genes. In juveniles, TC + C induced a higher proportion of females and higher cyp19a1a expression compared to CTE + C. HT induced changes in the CTE group by up-regulating testicular differentiation genes and down-regulating female promoting genes, which did not occur in the TC group. Juveniles from TC + C group presented a higher proportion of females with higher E2 and cyp19a1a than CTE + HT. Fish from the CTE + HT group showed a higher percentage of males with highest T and amh. These findings indicate that daily TCs during larval development promote ovarian differentiation and diminish the masculinizing effects of HT.